The first law of thermodynamics is that energy can be neither created or destroyed. We are made from energy - in fact, everything is. When our bodies die and return to the earth, they feed the soil and, in turn, the trees and the plants, etc etc. Ashes to ashes, energy to energy.
Now, time is also an entity of some kind. There is the age old question one person might ask another, in a metaphorical or facetious kind of way: you're sitting on your white-painted wicker furniture on the porch at dusk one summer evening, and you look at the person sitting next to you and realize that the summer is almost over, and you find yourself asking: 'where does the time go?' But when you take a closer look at that question, it isn't just a meaningless question without an answer. Time has to go SOMEWHERE, because it is someTHING. Just as gravity is an invisible force that holds us down on our planet, time is an equally invisible-but-none-the-less real force that continues to push all living things forward, completely unstoppable and uncontrolled.
So where does it go? It can't just fall into the abyss because, as we concluded, energy - something everything is made up of - can be neither created or destroyed. So it must, as the Taoists believe, be cyclical, not linear. Therefore, it must recycle itself.
The question becomes not "where does it go?", but 'what does it transform into?' The Mayan calandar stops at 2012, and this can be attributed simply to the fact that the ancient civilization could not be expected to document the future years of all eternity. But maybe they were onto something - maybe that is the end of time's cycle, and it will continue on again fresh and new starting that year. That is not to say there will be an apocolyptic ending, or that we will be able to sense a change. No; if time recycling is true and real, then it would just continue on like any other day, a new beginning but not one that is dramatic or life-changing or even noticeable.
There could be other theories or explainations. If energy is neither created nor destroyed, and whatever is within the human body is energy (call it your essence, your soul, your being, your consciousness, whatever), then something has to happen to that soul (being, essence, et all). Perhaps there is a heaven, but that doesn't seem entirely plausible. What could happen is as time recycles, so do souls - into new bodies, new lives, with new purpose and new lives. Or maybe we live the same life over and over and over again without change, just in different dimensions, with start and end dates randomly selected. Or perhaps we live similar lives to the one preceding it, but they are never completely identical, because even in nature, finding something exactly the same as another is not very common.
That just might be it.